1. Field of the Invention
This invention relates to a balancer for a reciprocating machine including a crank, such as a crank press, for balancing unbalanced inertia forces in the reciprocating machine.
2. Description of the Prior Art
Crank mechanisms are generally classified into two types: one type being characterized in that a crank pin off-centered with respect to a crank shaft is supported by a crank arm, and the other being characterized in that a portion of a crank shaft is made eccentric to define an eccentric shaft portion. In either type, an eccentric rotary portion of the crank mechanism is connected via a connecting rod with a reciprocating member to convert rotary motion into reciprocating motion.
A crank press is known to be a typical reciprocating machine employing such crank mechanisms. When such a machine is operated at a high speed, vibration arises owing to unbalanced inertia force. A balancer normally is used as a measure to prevent occurrence of such vibration. A typical crank press employing a conventional balancer will now be described with reference to FIG. 5 which is a simplified block diagram and FIG. 6 which is a diagram illustrative of a motion mechanism.
In these drawings, reference numeral 1 designates a drive motor, 2 a belt, 3 a pulley, 4 a crank shaft, 5a and 5b crank arms, 6a and 6b connecting rods, 7 balancers, 8 a slider on which an upper metal mold 9 is secured, 10 a lower metal mold, 11a and 11b journal bearings, 12 an upper casing, 13 a machine body frame, and 14a and 14b guides for a reciprocating member. The rotary motion of the motor 1 is transmitted via the belt 2 and the pulley 3 to the crank shaft 4 to rotate the crank arms 5a and 5b, whereby such rotation is converted into reciprocating motion of the balancers 7 and the slider 8 in the direction of an arrow Z.
Symbolizing the parameters shown in the drawings as follows:
r.sub.1 : the crank radius for the slider PA1 r.sub.2 : the crank radius for the balancer PA1 l.sub.1, l.sub.2 : the length of each connecting rod PA1 .lambda..sub.1 =r.sub.1 /l.sub.1, .lambda..sub.2 =r.sub.2 /l.sub.2 PA1 .omega.: the angular velocity of rotation PA1 t: time PA1 .theta.=.omega.t PA1 m.sub.1 : the mass of the slider PA1 m.sub.2 : the mass of the upper metal mold PA1 M.sub.1 =m.sub.1 +m.sub.2 : the sum of mass of the reciprocating sections PA1 M.sub.2 : the sum of mass of the balancers 7 PA1 .alpha..sub.1 : the acceleration of the composite center of gravity of the slider 8 and the upper metal mold 9,
the unbalanced inertia force (the vibromotive force) of the crank press is due in part to the mass M.sub.1 of the reciprocating sections such as the slider 8 and otherwise to an unbalance in the gyrating (rotating) mass of the crank arms r.sub.1 and r.sub.2 and the like. Among the above the unbalance of the gyrating mass is balanced by balance weights (not shown) attached to the sides opposite to the crank arms r.sub.1 and r.sub.2. Hence, this unbalance is not considered in the description herein.
On the other hand, the vibromotive force F.sub.1 due to the mass M.sub.1 of the reciprocating sections, such as the upper metal mold 9 and the slider 8, is expressed as EQU F.sub.1 =-M.sub.1 .alpha..sub.1 .apprxeq.-M.sub.1 r.sub.1 .omega..sup.2 (cos .theta.+.lambda..sub.1 cos 2.theta.) (1)
In contrast with the above, each balancer 7 is connected with a respective crank arm 5b which has a phase difference of 180.degree. with respect to the crank arms 5a of the slider 8, hence, the vibromotive force F.sub.2 of the balancers 7 is expressed as EQU F.sub.2 =M.sub.2 r.sub.2 .omega..sup.2 (cos .theta.+.lambda..sub.2 cos 2.theta.) (2)
In each of the above equations (1) and (2), the quadratic vibromotive force represented by the second term is remarkably small as compared with the linear vibromotive force represented by the first term, and hence, no description is given here with respect thereto.
Accordingly, the balance equation between the linear vibromotive forces F.sub.1 and F.sub.2 as derived from the equations (1) and (2) is: EQU -M.sub.1 r.sub.1 .omega..sup.2 cos .theta.+M.sub.2 r.sub.2 .omega..sup.2 cos .theta.=0 (3)
Dividing both sides by .omega..sup.2 cos .theta., the following is obtained: EQU -M.sub.1 r.sub.1 +M.sub.2 r.sub.2 =0 (4)
Thus, an unbalanced amount f to be borne by the adjusting weight of the balancers 7 is expressed as EQU f=M.sub.2 r.sub.2 =M.sub.1 r.sub.1 ( 5)
From the above expression (5) it becomes apparent that as the sizes of the crank radiuses r.sub.1 and r.sub.2 are determined, the weight (the mass) of the balancers 7 can easily be determined and the linear vibromotive force can perfectly be balanced.
In the press, however, the features of the metal molds 9 and 10 generally vary, depending upon the specification of finished products, and their weight also varies. Thus, in the conventional press, it is necessary to perform manual adjustment such that the weight of the balancers 7 (or the weight of the slider 8) matches the weight (or the mass) of the upper metal mold 9.
In this connection, since the balancers 7 are housed inside the upper casing 12 of the machine body frame 13, large amounts of manpower and of time are required for the operation of attaching/detaching the balance weight, and production efficiency is lowered.